Flywheel energy storage systems have emerged as an alternative to electrochemical batteries for storing energy with many advantages including higher reliability, longer life, lower or no maintenance, higher power capability and environmental friendliness. Flywheel energy storage systems store energy in a rotating flywheel that is supported by a low friction bearing system inside a chamber. The chamber is usually evacuated to reduce losses from aerodynamic drag. The flywheel is accelerated for storing energy and decelerated for retrieving energy through use of a motor/generator attached to the flywheel. Power electronics maintain the flow of energy in and out of the system and can prevent power interruptions or alternatively manage peak loads. When used in uninterruptible power supplies, flywheel systems employ a continuously rotating flywheel that is readily available to be slowed for providing power during an interruption of primary power.
Several possible flywheel system configurations can be constructed including permanent magnet excited and field coil excited. Permanent magnet type motor generators utilize permanent magnets to provide field flux. They can efficiently provide power instantaneously during an interruption because they do not require power for generation of flux and the flux is always produced. Many designs have extremely low inductance, which beneficially limits any voltage drop from armature reaction when high power current is extracted. Unfortunately, the voltage from permanent magnet motor-generators drops as the flywheel speed slows. To extract a large portion of the energy in flywheels to supply a load, complex and costly electronics have always been required.
Field coil excited flywheel systems utilize current applied to a field coil to generate field flux. Because the field coil current can be controlled, the current can be increased as the flywheel speed slows. This allows extraction of significantly more useable energy from the flywheel. However, such designs have higher losses in order to provide field coil excitation as well as increased magnetic losses in many cases. They also provide less than instantaneous full power when operated with a reduced field coil energization during standby for reducing losses. A time lag occurs from sensing a power interruption and then increasing the field current in order to generate full power to supply the load. The inductance time constant of the field coil itself accounts for much of the delay. Such flywheel systems also require both a complex and costly power monitoring system and may not provide complete protection against power interruptions.